1. Field of the Invention
The present invention relates to waste-spark ignition of an internal combustion engine (the term "waste-spark" is herein used to indicate a technique of firing two spark plugs at the same time by means of one ignition coil). More particularly, the present invention relates to a method of and apparatus for producing spark for such waste-spark ignition, which is capable of preventing or suppressing occurrence of a difference in the spark plug discharge characteristic between the cylinders due to wear of the electrodes of the spark plugs.
2. Description of the Related Art
In recent multi-cylinder engines is used an electronic ignition system which provides sparks to cylinders without use of a distributor. Enumerated as such an electronic ignition system are (1) first one wherein cylinders which differ in operating cycle phase by 360 degrees are and when one of the paired cylinders is on compression stroke (thus, the other cylinder is on exhaust stroke) two spark plug provided to the respective cylinders are fired at the same time, i.e., a so-called simultaneous ignition or waste-spark type, and (2) another one wherein the spark plugs are fired independently when the respective cylinders are on compression stroke. The first mentioned, waste-spark type of the above mentioned two ignition systems can provide sparks to two cylinders by means of one ignition transformer or coil and therefore superior in cost to the second mentioned type.
FIG. 6 shows a prior art waste-spark or double ended distributorless ignition system 100 wherein a primary winding 2 of an ignition coil or transformer 1 is connected at an end thereof to a direct current source E by way of a key switch or ignition switch K and at another end thereof to a collector of a transistor 4. A secondary winding 3 is connected at opposite ends thereof to spark plugs P1 and P2 provided to rective cylinders N1 and N2 of an engine N, which differ in operating cycle phase by 360 degrees.
Although the actual number of cylinders of the engine N is not always two but six for instance, explanation for other cylinders is omitted for brevity.
With this circuit, an ignition operation is carried out at a predetermined ignition angle in response to the output of a crank angle sensor or the like. Specifically, when the ignition switch K is turned on to connect the direct current source E to the primary winding 2, the voltage at the ignition signal line Sig of the ECU (engine control unit) 5 rises up to turn on the transistor 4, thus allowing current to flow from the direct current source E to the primary winding 2. Thereafter, the voltage at the ignition signal line Sig is lowered at a predetermined ignition timing (e.g., on compression stroke of the cylinder N1) by means of the ECU 5, a high voltage is induced in the secondary winding 3 to cause the first and second spark plugs P1 and P2 to produce sparks, thus igniting the air-fuel mixture in one of the cylinders (e.g., cylinder N1) on compression stroke while producing waste-spark in the other cylinder (e.g., cylinder N2) on exhaust stroke.
However, in case the circuit shown in FIG. 6 is used, the polarity of the high voltage induced in the secondary winding 3 is constant. Thus, negative polarity spark is always produced by the first spark plug P1 in which the more negative potential is caused in the center electrode P1i than in the ground electrode P1o which is grounded, whereas positive polarity spark is always produced by the second spark plug P2 in which the more positive potential exists in the center electrode P2i than in the ground electrode P2o.
For this reason, the two spark plugs P1 and P2 differ in the negative potential electrode which is more liable to wear away than the positive potential electrode since it is impacted by positive ions caused by the sparks. Thus, wear of the center electrode P1i is mainly caused in the first spark plug P1, whereas wear of the ground electrode P2o is mainly caused in the second spark plug P2.
In the meantime, in case ignition is carried out with the above described waste-spark method, sparks are provided to both of the cylinders N1 and N2 on both of their compression and exhaust strokes, but most wear of the electrodes is caused by spark on compression stroke. It is assumed that the electrodes are liable to wear away on compression stroke since on compression stroke the electrodes are subjected to a high pressure and exposed to a high temperature by being surrounded by the flames during spark discharge.
Further, more marked wear results in case natural gas or the like fuel that burns at high temperature is used or high voltage is used to produce the spark for lean-burn.
In this instance, increase of the spark gap due to wear of the electrodes is largely influenced by the wear of the center electrode P1i which is smaller in volume. In contrast to this, the ground electrode P2o is larger in volume so its wear can increase the spark gap at only a small rate. For this reason, more increase of the spark gap is caused in the first spark plug P1 as eared with the second spark plug P2, thus increasing the voltage necessitated for producing the spark.
However, in case the spark plugs are used in an engine of the type in which the ground electrodes are liable to have a high temperature to be oxidized, there may occur such a case in which the wear of the ground electrodes is accelerated by the oxidization. In such a case, increase of the spark gap is largely influenced by the wear of the ground electrodes.
In any event, the difference in the wear of the spark plug electrode occurs between the cylinders to cause a difference in the spark plug discharge characteristic between the cylinders N1 and N2. Further, such difference in wear causes one of the spark plugs to increase in spark gap excessively, thus causing the life of one of the spark plugs to expire faster than the other, e.g., the life of the first spark plug P1 expires faster than the second plug P2.
In this connection, it is considered to use spark plugs of different part number for the spark plugs P1 and P2 to be put to the engine N, i.e., to use a spark plug with a wear-resistant center electrode P1i for the first spark plug P1 and a spark plug with a wear-resistant ground electrode P2o for the second spark plug P2. However, this is not desirable since this will cause the part numbers of spark plugs to be controlled, to be doubled and furthermore will induce erroneous installation of spark plugs.
Thus, in order to prevent the wear of the electrodes, it has been practiced to use such spark plugs P1 and P2 in which all of the center electrodes P1i and P2i and the ground electrodes P1o and P2o have welded thereto wear-resistant electrode chips. However, one of the wear-resistant electrode chips which are made of an expensive metal such as platinum, iridium, rhodium and rhenium, e.g., the wear-resistant electrode chip attached to the ground electrode P1o of the spark plug P1 remains unchanged without having almost any wear even when the chip of the center electrode P1i has worn away to cause expiration of the life of the spark plug P1, so one of the wear-resistant electrode chips is wasted.